1. Field of the Invention
The present invention relates to a process for conducting catalytic multiphase reactions in a tube reactor. More particularly, the invention relates to a method of preparing vinyl esters by reaction of carboxylic acids with acetylene. Furthermore, the invention relates to the use of the vinyl esters.
2. Description of the Background
Vinyl esters are building blocks for the preparation of copolymers. Vinyl esters of particular industrial importance include those of tertiary C10 acids and also those of tertiary C9 acids. These vinyl esters can be copolymerized with vinyl acetate and then serve as a component for paints. Other vinyl esters of importance include those of 2-ethylhexanoic acid, of isononanoic acid, but also of lauric and stearic acids, for example for the production of adhesives.
The reaction of carboxylic acids with acetylene to form vinyl esters has been known for a long time (e.g. G. Hxc3xcbner, Fette, Seifen, Anstrichm, 68, 290 (1966)), and a review of currently used processes may be found, for example, in Ullman""s Encyclopedia of Industrial Chemistry, Fifth Completely Revised Edition, Volume A2, 430-434.
The reaction of acetylene and vinyl esters can be described by the reaction equation:
Rxe2x80x94COOH+C2H2xe2x86x92Rxe2x80x94COOxe2x80x94CHxe2x95x90CH2.
As catalyst, use is usually made of the zinc salt of the carboxylic acid to be reacted, dissolved in excess carboxylic acid (DE 1238010, DE 1210810). In another method, the reaction is carried out in a paraffinic solvent which has a boiling point higher than the reaction temperature to be employed (e.g paraffin oils) and the concentration of carboxylic acid is to be kept below 5% by weight and the zinc content is to be kept in the range from 0.5-5% by weight, based on the reaction solution. The temperatures employed are generally above 200xc2x0 C., typically from 220-295xc2x0 C. A considerable molar excess of acetylene over the carboxylic acid to be vinylated has to be employed; values of 5-20 mol. of acetylene per mol. of carboxylic acid to be reacted are typical. This means a high circulation of acetylene with the vinyl ester and unreacted carboxylic acid being discharged with the waste gas as a function of their partial vapor pressure, subsequently condensed and separated from one another by distillation.
The formation of low-boiling by-products such as acetaldehyde can lead to resinification of the catalyst. Furthermore, the formation of high boilers due to polymerization of the vinyl ester formed and also the formation of carboxylic anhydrides which no longer participate in the reaction can lead to a reduction in conversion. This can be corrected only by discharging part of the reaction solution or the zinc salt melt, which in turn requires introduction of fresh catalyst. In the examples in DE 1 237 557, it can be seen that at an appropriately high molar excess of acetylene over carboxylic acids of from 5:1 to 20:1, selectivities of from 74.3 to at most 96.1% are achieved. This corresponds to formation of from 3.9-25.7% of by-products.
In engineering terms, the preparation of vinyl esters is a two-phase reaction in which the acetylene forms the gas phase, and the catalyst phase, the carboxylic acid to be reacted, the vinyl ester and any inert solvent form the liquid phase. In order to achieve high conversions, it is, therefore, necessary to employ measures to bring the phases into intimate contact with one another. Such gas-liquid reactions are customarily carried out in bubble columns or stirred reactors. Trickle reactors are also mentioned in the literature. High pressures are specifically not wanted in reactions with acetylene, since acetylene tends to undergo autodecomposition and a gauge pressure of 300 mbar (absolute pressure=1.3 bar) must not be exceeded in the case of pure acetylene. This considerably narrows the choice of reactor. Thus, only short bubble columns or stirred reactors having a small height can be used for the vinylation, since even the static pressure of the liquid can require an acetylene admission pressure of greater than 300 mbar. In addition, the high acetylene circulations which may be necessary require continual recompression of the acetylene.
An alternative is the use of more highly compressed acetylene, in which case the decomposition limit has to be lowered by addition of large proportions of inert gases such as nitrogen. This measure leads to problems in the removal of the tailgas and is also not unproblematical in terms of safety.
For this reason, other processes which avoid or reduce these drawbacks have been developed. Thus, EP 0 622 352 uses platinum metal complexes (in particular those of ruthenium) as catalysts for the preparation of vinyl esters, by means of which the reaction temperature was able to be reduced significantly. However, this process is also preferably carried out at a molar ratio of acetylene/carboxylic acid of from 1.5:1 to 10:1 and at acetylene pressures of from 15-20 bar in order to achieve satisfactory space-time yields. The high price of noble metal compounds necessitates complete circulation of the catalyst without it being decomposed or lost by unintended discharges. A further example of the ruthenium-catalyzed preparation of vinyl esters is given in U.S. Pat. No. 5,430,179.
A completely different process for preparing vinyl esters is transvinylation. Here, an industrially readily available vinyl ester such as vinyl acetate is reacted with the acid to be vinylated in the presence of noble metal catalysts such as Li2PdCl4, forming an equilibrium mixture of vinyl acetate, acetic acid, the carboxylic acid to be reacted and its vinyl ester. Problems here are the difficulty and expense of separating the equilibrium mixtures, the price of vinyl acetate and the formation of contaminated acetic acid.
Taking into account all advantages and disadvantages, the vinylation of carboxylic acids using acetylene in the presence of zinc salts as catalysts, i.e. a multiphase reaction, is on balance the most economical and industrially simplest process.
In the following, the term multiphase reactions is used to describe reactions which proceed with participation of two or more immiscible or only partially miscible fluid phases. This encompasses, for example, reactions between a gas phase and a liquid phase (gl) as in the reaction of acetylene with a carboxylic acid, but also reactions between two liquid phases which are immiscible or have a miscibility gap under the prevailing reaction conditions (ll) and reactions in which both two liquid immiscible or only partially miscible phases and a gas phase participate (gll).
Examples of industrially important gas-liquid reactions (gl) are, apart from the reaction of acetylene with carboxylic acids under consideration here, hydrogenations using homogeneously dissolved catalysts, oxidations using air or oxygen and the hydroformylation of olefins.
Multiphase reactions are generally associated with a series of problems which makes their engineering design significantly more difficult than is the case for simple homogeneous reactions. A few typical problems are described below:
In all cases, the materials have to be brought into very intimate contact with one another in order to minimize the problem of mass transfer: it is necessary to generate a mass transfer area a, between the phases which is as large as possible. On the other hand, the phases have to be able to be separated easily again after the reaction is complete: excessive mixing can lead to problems here. In the presence of two liquid phases, emulsion formation can occur, while in the case of gas-liquid processes it is possible for foaming to occur. In the case of the 3-phase processes mentioned, it is even possible for all of the problems to occur simultaneously.
Apart from a high mass transfer area as, a very high mass transfer coefficient kit should be achieved in all multiphase reactions. Overall, the KLA value, i.e. the product of kl and as in the mass transfer equation
j=kl*as*(C*xe2x88x92C)
where:
j [mol/s]: the molar flow of reacting component through the phase interface (e.g. entry of acetylene into the catalyst phase),
kl [m/s]: mass transfer coefficient,
as [m2]: phase interface area in the reactor,
C* [mol/m3]: maximum solubility of the starting material in the second phase (e.g. acetylene in the catalyst phase) and
C [mol/m3]: actual concentration of the starting material which in turn is coupled to the reaction rate,
should be a maximum.
In view of the above, there is a need for a process for carrying out multiphase reactions which avoids the abovementioned disadvantages and can also be implemented in a simple manner.
Accordingly, one object of the present invention is to provide a process for conducting multiphase reactions which is suitable, in particular, for the preparation of vinyl esters by catalyzed reaction of carboxylic acids with acetylene.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for conducting catalytic multiphase reactions, comprising:
conducting the catalytic multiphase reactions in a tube reactor, wherein the catalyst is present in the continuous phase and at least one starting material is present in a dispersed phase and the loading factor B of the tube reactor is equal to or greater than 0.8.